1. Technical Field
The present invention relates in general to linking in a telecommunication system and in particular to a method for partial reporting of missing sequence numbered information frames in an ascending sequential order by use of selective reject SREJ response frames. Still more particularly, the present invention relates to a method and system for continuous link establishment when out-of-sequence information frames are transmitted between receive and transmit terminals within a telecommunication or information handling system.
2. Description of the Related Art
Today, most of the transmission between telephone exchanges is digital, but the transmission and the signaling between a local exchange and a customer is still analogue. Because different users have different requirements (telephony, fax, data) in the existing analogue network, it is necessary to use separate telephone lines with separate characteristics for the different users. To solve this problem a new universal interface has evolved. This new network evolution is called the Integrated Services Digital Network (ISDN). ISDN is a versatile network in which all kinds of information technology are transmitted in a digital format between terminals. The ISDN digital access provides the end users with the possibility of initializing end-to-end digital connections that can support a variety of services, such as: high quality speech, audio, video telephony, video conference, as well as fax, data and image transfer capabilities.
To ensure that voice and signaling or data and signaling information can be transmitted simultaneously in ISDN with no mutual reference, multiple channels are provided on an ISDN access and are defined by the International Telecommunication Union (ITU) in a series of protocol specifications. These channels are identified as B-channels and D-channels respectively. The B-channels are used for information transfer, such as speech, text, data and images. The D-channel is used for call control signaling procedures and may also be used to carry packet-switched data. The call control signaling information on the D-channel is used to establish, supervise and release the circuit-switched connections being requested for.
Unlike the B-channels, which function as xe2x80x9cpipes,xe2x80x9d the D-channel is associated with higher level ITU protocol specifications defined by an Open System Interconnection (OSI) model. The OSI model has 7 protocol specification layers in which layers 2 and 3 of the model form the packet-switched connections associated with the D-channel. Within the layer 3 protocol specifications for use on the D-channel is a call-control protocol component. This layer 3 signaling protocol is transferred on the D-channel using the layer 2 protocol referred to as the Link Access Procedure-D-channel (LAPD.) Although the layer 3 protocol takes care of most ISDN signaling requirements, the LAPD layer 2 protocol utilizes frames that play a very important role in terms of low-level signaling to ISDN devices. These LAPD frames contain the information to ensure that incoming calls are routed to the appropriate ISDN device, and pass the addressing information that distinguishes ISDN devices on a single line from each other.
For continuous information to be transferred between two terminals, sequence numbered information frames are sent along the D-channel from a transmit terminal in a sequencing order to a receive terminal. The sequence numbered information frame has a predetermined byte structure recognized at the receive terminal in accordance with the ITU protocol specification. However, a problem arises due to such effects as microwave fadeout and the like, in that one or more sequence numbered information frames may be lost during transmission resulting in a loss of the sequencing order. In the past this problem has been overcome by constructing a selective reject response (SREJ) frame upon receiving any out-of-sequence numbered information frames. The SREJ frame lists all missing sequence numbered information frames starting at the first missing sequence numbered information frame and ending with the last missing sequence numbered frame prior to the received sequence information frame and re-transmitting this complete list. However, this solution does not address the problem that during a real-time-critical period in the error recovery of a LAPD link, the terminal receiving the SREJ frame must process the SREJ frame which may contain a list of sequence number frames which may or may not be in order. This possible out-of-sequence list requires time consuming methodologies to perform a sort of the sequence numbers in the SREJ frame upon reception.
Therefore, there is a need to minimize real-time processing effort yet at the same time providing complete coverage of possible error conditions by always constructing a SREJ frame containing a list of missing sequence numbered information frames in ascending sequential order thereby eliminating the need to perform sorting algorithms. The subject invention herein solves this problem in a new and unique manner that has not been part of the art previously.
It is therefore an object of the present invention to provide a method and system for continuously linking within a telecommunication system or information handling system.
It is yet another object of the present invention to provide a method and system for minimizing real-time processing effort yet at the same time providing complete coverage of possible error conditions within a telecommunication or information handling system.
It is still yet another object of the present invention to provide a method and system for reporting and re-transmitting missing sequence numbered information frames that are out of sequence always in an ascending sequential order within a telecommunication or information handling system.
The foregoing objects are achieved as is now described. A method and system for sequential ordering of missing sequence numbered information frames within a telecommunication system using selective reject (SREJ) frames is disclosed. Upon receiving a sequence numbered information frame from a transmit terminal, a receive terminal determines if it is out of sequence. If the sequence numbered information frame is out of sequence, the receive terminal further checks the sequence numbered information frame to see if a poll bit has been set. If the poll bit is not set, the sequence numbered information frame further checks to see if the sequence numbered information frame is new or retransmitted. If the sequence numbered information frame is new, a list of missing sequence numbers is constructed in ascending sequential order beginning with the missing sequence number one greater than the largest number in the receive queue and ending with N(S)xe2x88x921. If the sequence numbered information frame is retransmitted, a list of missing sequence numbers is constructed in ascending sequential order beginning with the next smaller missing sequence number in the receive queue plus one and ending with N(S)xe2x88x921. In either case the ascending sequential order is constructed with respect to the transmit window and the modulo 128 sequence numbering scheme of the LAPD protocol. Thereafter, a select reject frame (SREJ) is sent to the transmit terminal containing a list of the out-of-sequence numbered information frame(s) for re-transmittal.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.